Conventional technologies for providing high power fiber laser sources for future free space telecommunication systems are still limited by several technical difficulties. Specifically, in the processes of producing high-energy pulses, current techniques and systems tend to sharpen the temporal responses and create a high peak region in the leading edge of the pulse. The high power leading edge causes potential optical damages of the internal components of the laser source and further reduces the transmission distance due to a non-transform limited pulse shape. The sharp leading edge of the laser pulses when projected into the optical components in an optical transmission system generates a high-energy shock over very short time duration. This instantaneous energy shock may well be over the damage threshold of the components, e.g., the damage thresholds of the photo-detectors and optical filters, and thus cause instantaneous system malfunction and that often leads to permanent damage. Moreover, for an optical communication system, the higher power density caused by these sharpened temporal responses and higher leading edge of the laser pulse are often detrimental to the detection subsystem due to the energy transfer to the Stokes lines now provided with a stronger fiber amplifier.
Another technical difficulty that often limits the capabilities of a fiber laser source is related to the laser chirp due to modulation and self phase modulation (SPM) that leads to distortions of the laser pulses. As that shown in FIGS. 1A and 1B, the time dependent frequency of a laser pulse in the frequency domain depends on the deviation of the pulse power. The relationships can be expressed by the following equations according to F. Koyama as disclosed in “Frequency Chirping in External Modulators” published in the Journal of Lightwave Technology, Volume 6, No. 1, 1988.
                                          f            ist                    ⁡                      (            t            )                          =                              f            0                    +                                    α                              4                ⁢                                                                  ⁢                π                                      ⁢                                                            ∂                                      S                    ⁡                                          (                      t                      )                                                                      /                                  ∂                  t                                                            S                ⁡                                  (                  t                  )                                                                                            α        =                  2          ⁢                                          ⁢          S          ⁢                                                    ∂                Φ                            /                              ∂                t                                                                    ∂                S                            /                              ∂                t                                                        
The equations show that as the steepness of the pulse in time domain increases the frequency shift also increases. Under practical operational conditions, the optical signals are generally transmitted over a dispersive medium. As a result, when the signals are transmitted over a distance over a dispersive medium, more distortions are increasingly introduced into the optical signals and as the signals are spread over a broadened frequency range that usually also generates optical signals with an unsymmetrical spectrum. Therefore, the pulse distortions as encountered in the practical operational conditions described above, further causes the steepness of the laser pulses thus worsening the problems resulted from the high power at the lead-edging of the laser pulses.
Many laser source technologies are known and available for those of ordinary skill in the art. For application to modern telecommunication systems, due to the requirements to have well-defined pulse shape and large extinction ratios, the traditional Q-fiber lasers as disclosed by several prior art publications are no longer useful. These publications include: J. Yang, et al., “Wide band erbium doped fiber ring laser using switchable fiber Bragg gratings,” SPIE 4594, 282 (2001); G. P. Lee, et al., “980 nm diode pumped Er/Yb doped Q switch fiber laser.” Electron. Lett. 31(21), 1836-1837(1995); G. P. Lee, et al., “Q switched erbium doped fiber laser utilizing a novel large mode area fiber.” Electron. Lett. 33(5), 393-394(1998); R. J. Mears, et al., “Low threshold tunable CW and Q-switch fiber laser operating at 1.55 micon,” Electron. Lett. 22(3), 159-160(1986); P. Mylinski, et al., “High power Q switched erbium doped fiber laser,” IEEE J. Quentum Electron. 28, 371-377 (1992); F. Sequin, et al, “Diode pumped Q switch laser,” Opt. Eng. 32(9), 2036-2041 (1993); M. Sejka, et al., “High repetition rate Q switch erbium doped fiber ring laser,” Optical Fiber Technology 1, 167-170 (1995); and A. Chandonnet, et al., “High power Q switched erbium fiber laser using an all fiber intensity modulator,” opt. Eng. 32(9), 2031-2035 (1993).
More technologies to improve the performance and power of the laser sources were also investigated and published. However, the above-mentioned technical difficulties are not yet resolved by prior art investigations as disclosed in the many publications in the field of fiber laser researches. These disclosures include different amplification schemes used as alternative techniques to produce high energy and high repetition rate pulses via a gated amplification. The results are published by B. Desthieux, R. L. Laming, and D. N. Payne, entitled “111 kW Pulse amplification at 1.5 micron,” Appl. Phys. Lett. 63(5), 585-588 (1993); D. Taverner, et al., “Generation of high energy pulses using a large mode area erbium doped fiber amplifier,” Proc. CLEO'96, 496-497 (1996); and D. Gapontsev, et al., “25 kW peak power, wide tunable repetition rate and pulse duration eye safe MOPFA laser,” Proc. CLEO'96, 209-210 (1996). Techniques by using multiple isolated gain stages were also investigated and published by A. Galvanauskas, et al., “Compact ultra high power laser system,” SPIE 2377, 117-126 (1995); D. Taverner, et al., “158 micron Joul pulses from a single transverse mode, large mode area erbium doped fiber amplifier,” Opt. Lett. 22 (6), 378-380 (1997); and D. Rafizadeh, et al., “Kilowatt pulses from a single mode erbium doped amplifier,” Electron. Lett. 317-318 (1994). However, these techniques have not been successful to provide solution for the technical difficulties caused by sharpened temporal responses in the leading edge of high power laser pulses and different pulse distortions caused by laser chirp difficulties. FIGS. 1C and 1D is an example extracted from Opt. Lett. 22, 378 (1997) to illustrate the pulse shape evolution in amplifiers over test points 1,2, and 3. The sharp leading edges as discussed above are clearly shown in these figures. As shown by FIGS. 1C and 1D, the prior art technologies as those available to those of ordinary skill in the art, have not provided system configurations and methods to overcome these difficulties.
Therefore, a need still exists in the art of fiber laser source design and manufacture to provide a new and improved configuration and method to provide high power laser source with compact configuration, low power consumptions and high quality pulse shapes with high extinction ratio and overall power utilization efficiency.